The electrophotographic image forming apparatus has, in general, been used for a document copier, a laser beam printer and so on. In such a conventional electrophotographic apparatus, a corona discharger has been broadly used for electrostatically charging a photoconductor (photosensitive element) which is an object to be charged. The corona discharger comprises a fine wire and a shielding electrode. A high voltage, such as 4 to 5 kV, is applied to the wire. Thus, the photoconductor is electrostatically charged evenly by discharge between the wire and the photoconductor. Furthermore, another corona discharger, which has a grid disposed between the wire and the photoconductor, is used for making the charge distribution of the photoconductor more even. Such a corona discharger is called Scorotron and widely used now.
The Scorotron, however, needs an electric power supply which can supply a high voltage of several kilovolts for making the discharge stable. Furthermore, a lot of ozone, which is injurious to the human body, is produced during the discharge. Therefore, an ozone treatment apparatus is necessary. Or, the photoconductor is deteriorated by ozone.
On the contrary, another method or apparatus has been proposed for reducing ozone at a minimum. In the method or apparatus, a conductive charging member contacts the surface of the photoconductor. The discharge occurs between the charging member and the surface of the photoconductor, so that the photoconductor is directly charged. The discharge is kept at a minimum which is necessary for electrostatically charging the photoconductor. As a result, the amount of ozone, which is produced during the discharge, can be reduced.
There are many examples of the apparatus for directly charging the photoconductor by contacting the surface thereof. Publication gazette of Japanese Examined Patent Application Sho 62-11343 discloses the use of a conductive elastic roller as a charging member. Publication gazette of Japanese Unexamined Patent Application Sho 56-147159 discloses a method for using a fur brush (fiber brush). From the point of view of producing an electric field for generating the discharge, Publication gazette of Japanese Unexamined Patent Application Sho 58-194061 discloses the method for applying DC voltage to the charging member. U.S. Pat. No. 4,851,960 discloses the method for applying superposed DC voltage and AC voltage to the charging member.
In the method of using the fur brush, the contact condition of the fur brush and the surface of the photoconductor is unstable, so that the charge distribution on the surface of the photoconductor will be uneven. Furthermore, the fur of the brush will be deteriorated or will lie flat over time passes, so that charging will become unstable.
On the other hand, in the method of using the conductive elastic roller, the roller contacts the photoconductor more stably and evenly than in the case of using the fur brush. Thus, the deterioration of the roller becomes smaller. However, in the method of using the conductive elastic roller, unevenness of the charge distribution will occur owing to surface roughness or unevenness in resistance of the roller. Comparing the case of applying DC voltage to the roller to the case of applying the superposed DC and AC voltages to the roller, charge distribution in the latter case is flatter than that in the former case, and the tolerance of the charge in the latter case is larger than that in the former case. However, when the AC voltage is applied, vibration electric field is generated between the conductive elastic roller and the photoconductor, so that noise called charge noise occurs. Such a noise is governed by the frequency of the AC voltage which is applied to the conductive elastic roller. The noise becomes a problem when the frequency of the noise is in a region of audible frequency (20 to 2000 Hz, especially 200 to 2000 Hz). For preventing the audible noise, it is necessary to make the frequency of the AC voltage smaller than 200Hz or alternatively larger than 2000 Hz. When the frequency of the AC voltage is made larger than 2000 Hz, the voltage is largely attenuated in the charging member, and it becomes very ineffective. When the frequency of the AC voltage is made smaller than 200 Hz, frequent unevenness of the charge occurs in a circumferential direction of the photoconductor.
When the frequency of the AC voltage is designated by "f" (Hz) and the moving speed (called process speed) of the photoconductor is designated by "V.sub.p " (mm/sec), the frequent unevenness of the charge with a pitch of V.sub.p /f mm occurs in the circumferential direction of the photoconductor. This phenomenon occurs for the following reasons. The above-mentioned vibration electric field is gradually attenuated in a separating region of the charging member and the photoconductor, where their surfaces are gradually getting separate from each other, and surface potential of the photoconductor will be converged to the DC voltage which is superposed on the AC voltage. At this time, the frequency of the applied AC voltage is finite, so that transition and reverse transition of the electric charge between the charging member and the photoconductor may not simultaneously occur when the charging is completed (namely when the surface potential of the photoconductor is converged). Accordingly, the charging will be stopped when the final transition or reverse transition occurs responding to a phase of the frequency of the AC voltage at that time. When charging is completed, the phase of the frequency of the AC voltage in the axial direction of the photoconductor is constant but varies in response to the position in the circumferential direction of the photoconductor. Thus, a striped pattern of unevenness in the charge distribution, which is synchronized with the frequency of the AC voltage and parallel to the axis of the photoconductor, occurs. The pitch of the stripes is V.sub.p /f mm. When the pitch of the stripes is wider than the minimum pitch (resolution) at which a developing device of the image forming apparatus can develop, a developed picture image will be inferior or defective. For preventing an inferior or defective image, it is necessary to make the frequency "f" of the AC voltage larger. For example, in an image forming apparatus having a printing speed of four sheets of Japanese JIS standard paper size of A4 in one minute (processing speed is 25 m/sec), it is necessary to make the frequency of the AC voltage above 100 Hz.
Similarly, in an image forming apparatus having a printing speed of 30 sheets in one minute (processing speed is 190 mm/sec), the frequency of the AC voltage is above 750 Hz. In this case, the above-mentioned problem of the charging noise occurs. In other word, an upper limit of the processing speed of the image forming apparatus is limited by the frequency region of the AC voltage In which the charging noise does not occur. As a result, it is difficult to make the printing speed much faster by using the method of superposing the DC voltage on the AC voltage. Furthermore, an AC electric power supply is expensive and has a large volume. Thus, it makes the size of the image forming apparatus larger and the cost of the apparatus higher.
On the contrary, when only DC voltage is applied to the conductive elastic roller, the printing speed of the image forming apparatus can be made faster, the size of the apparatus can be made smaller and the cost of the apparatus can be made lower. However, it has the above-mentioned disadvantage that the charge distribution becomes uneven.